Such a device is known in principle from DE 20 2006 005 643 U1, although this document does not disclose any details of a reception optical unit. The applicant of the present application has, however, already sold devices as described in DE 20 2006 005 643 U1 including a reception optical unit comprising a pinhole diaphragm and a lens element, under the product designation LS 840/880.
The known device, known as a laser scanner, is designed to measure a spatial region and/or an object three-dimensionally. Typical applications include the measurement of the interior of a large workshop, the measurement of large objects such as buildings or ships' hulls, or the forensic measurement of a crime scene. The known laser scanner has a measuring head that can be rotated about a vertical axis. The measuring head contains a rotor with a minor inclined by 45°, which is arranged opposite a light emitter and a light receiver. The light emitter generates an emission light beam, which is deflected with the aid of the rotating minor in such a way that a vertical scanning fan arises. A reception light beam reflected from an object is diverted via the mirror onto the light receiver. The distance between the measuring head and the object point at which the emission light beam was reflected is determined from the propagation time of the emission and reception light beams. Rotation of the measuring head about the vertical axis makes it possible to rotate the (vertical) scanning fan in azimuth, such that it is possible to measure practically the entire spatial region around the laser scanner.
DE 20 2006 005 643 U1 describes an advantageous combination of such a laser scanner with an image recording unit that is designed to record a color image of the spatial region to supplement the distance measuring values.
One known problem in the case of such a laser scanner or more generally in the case of a distance measuring device which evaluates the propagation time of an emission and reception light beam involves the large intensity differences which the reception light beam can have, depending on the respective measuring distance. The larger the distance between the light emitter/light receiver and the object, the lower the signal power with which the reception light beam reaches the light receiver. In order to enable a large measuring range, the light receiver must be able to process the intense-power reception signals from short distances, on the one hand, and the weak-power reception signals from large distances, on the other hand. If the reception power is too high, the light detector is overdriven, which in extreme cases can lead to destruction of the light receiver. Even if the light receiver is not destroyed or damaged, reliable distance determination is not possible at an excessively high reception signal power. By contrast, if the reception power is too weak, the reception signal can no longer be distinguished from unavoidable interference such as ambient and detector noise.
In the case of the known laser scanner, the light emitter is situated coaxially in front of the reception optical unit and shades the central region thereof. On account of this, the reception light beam reaches the reception optical unit only in the form of a light ring from which the central region is “cut out”. This light ring is imaged onto the pinhole diaphragm by a lens element. If the diameter of the imaged light ring is smaller than the diameter of the pinhole diaphragm, the light ring fits completely through the pinhole diaphragm. In this case, the entire reception light beam with the exception of the shaded central region can be incident on a light detector arranged downstream of the pinhole diaphragm. However, if the diameter of the light ring is larger than the diameter of the pinhole diaphragm, the pinhole diaphragm limits the quantity of light that reaches the light detector. On account of the imaging properties of the reception optical unit, the diameter of the light ring depends on the distance covered by the reception light beam. The diameter of the light ring is small for large distances. The diameter of the light ring increases toward short distances. Owing to this behavior, a reception light beam from a large distance is imaged completely (with the exception of the central shading by the light-impermeable element) on the light detector, while a reception light beam from a short distance reaches the light detector only in part. This compensates somewhat for the different reception signal powers depending on the distance.
It has been shown, however, that this type of compensation is not optimal, since the light ring can become so large toward shorter distances that, on account of the central shading, it is no longer possible for any reception light at all to pass through the pinhole diaphragm. Consequently, the measuring range of the known laser scanner is disadvantageously limited toward short distances.